Seismic Sequence Research Articles

Semi-automated template matching and machine-learning based analysis of the August 2020 Castelsaraceno microearthquake sequence (southern Italy)

The accurate characterization of microearthquake sequences allows seismologists to better understand the physical processes involved in earthquake nucleation and rupture propagation and to gain insights on fault geometry at depth. Standard procedures for seismic sequences analysis are based on manual detection and phase-picking, requiring a huge amount of work from expert seismologists, particularly in the case of microseismic events. Here we show how the investigation of a low-magnitude foreshock-mainshock-aftershock sequence, occurred in August 2020 close to Castelsaraceno village (southern Italy), greatly benefited from the application of a semi-automated template matching and machine-learning based workflow. The phase-picking was automatically performed through a deep-learning algorithm on 202 microearthquakes detected between July and October 2020, followed by an automatic multi-step absolute and relative earthquake location procedure. The 72 relocated events of the seismic sequence were clustered in time (7–12 August) and in a narrow range of depths (10–12 km). The Ml 2.1 foreshock doublet and the Ml 2.9 mainshock identified a persistent asperity. The joint analysis of aftershocks distribution, the mainshock focal mechanism, and the geology of the study area suggest the occurrence of the sequence along a NNE-SSW left-lateral, transtensional fault in the brittle portion of the crystalline basement.

Chapter 6. Seismic expression of North Sea Jurassic sequences

Abstract Many of the stratigraphic sequences recognized in North Sea Jurassic well sections correspond to mappable surfaces on seismic sections. Typically, however, sequences are only mappable seismically within individual sub-basins, and seismic correlation between sub-basins, or across highs, is generally impossible without independent control from wells. Particularly prominent seismic sequence boundaries occur at near-base J54 in the Inner Moray Firth (‘Intra-Oxfordian Event’) the Viking Graben (‘Top Heather’ in this area), base J62 (‘Top Heather’, Moray Firth), base J66 (‘Top Lower Hot Shale’, Inner and Outer Moray Firth), base J71 (East Shetland Platform), base J73 (‘Top Siltstone Member’, Moray Firth) and top J70/base K10 (‘Base Cretaceous Unconformity’ (BCU), basin-wide). The BCU is the most frequently mapped seismic horizon in the North Sea Basin in Jurassic–basal Cretaceous studies. This surface, at the base of the Cromer Knoll Group, separates synrift sediments from post-rift successions above and marks a major shift in the tectonic evolution of the North Sea Basin.

Physics-based parametrization of a FAS nonergodic ground motion model for Central Italy

We propose a new fully nonergodic ground motion model for Central Italy, which is one of the most sampled areas in the world after the occurrence of the last seismic sequences of 2009 and 2016–2017. The model predicts 69 ordinates of the Fourier Amplitude Spectrum in the magnitude range 3.2–6.5 and is constrained on a dense set of seismological and geophysical parameters (i.e. stress-drop Delta sigma, shear-wave velocity in the uppermost 30 m, VS,30 and high-frequency attenuation parameter at source {kappa }_{source} and site {kappa }_{0}) made available from a non-parametric generalized inversion technique (GIT). The aim of this work is to capture the underlying physics of ground motion related to different source energy levels, as well as to the crustal and geological structure of the region, thus providing less uncertain predictions. Calibration is performed using a stepwise regression approach which has the advantage of taking a more complex functional form (advanced model) when all physical parameters are known while returning a simpler form (base model) when physical data are missing. As a result, the advanced model reproduces the reference rock motion of the region in case the site additional proxies are set to their average values (VS,30 = 1100 m/s, {kappa }_{0}=15 ms). We show that the inclusion of the set of physically-based explanatory variables in the regression has a beneficial effect in constraining the uncertainty, leading to a reduction of the high-frequency variability of about 70% on the between-event and 35% on the site-to-site. This reduction can be viewed as the result of the combination of a more effective physical description through the incorporation of the additional proxies and a calibration embedded in a completely nonergodic framework.

Plumbing systems and associated seafloor fluid seepages in deep-water Nigeria: Factors controlling their architecture and cyclic evolution

A range of plumbing systems has been identified in deep-water Nigeria by interpreting multi-scale seismic data sets (reprocessed 3D exploration seismic reflection data and 2D very high-resolution near-bottom seismic reflection data) as well as sedimentological and geochemical data obtained from a long sediment core recovered by the seafloor drill rig MARUM-MeBo70. The plumbing systems are characterized by diverse fault networks and lie above tectonic features including thrust/fold structures and strike-slip faults linked to gravity-driven deformation caused by underlying over-pressured shales. The plumbing systems are associated with fluid seeping structures at the seafloor such as pockmarks, mud volcanoes and seafloor zonations colonized by living benthic macrofauna typical of active fluid flow. The comparison of seismic stratigraphic sequences with climate and environmental proxies determined for sediments including lithology, element ratios such as Ca/Fe and Zr/Rb, sedimentation rates, and planktonic foraminifera δ18O records shows a control of short-term (0.1–0.4 Myr) and long-term (around 1 Myr) sedimentary cycles. During short-term fluctuations, the recurrence of fluid seeping structures is controlled by the lithology (coarse-grained versus fine-grained sediments) linked to glacial-interglacial fluctuations and a monsoon regime. Conditions favorable for fluid flow are related to the deposition of coarse-grained layers, glacial periods, sea-level lowstands, and low sedimentation rates. We hypothesize that cyclic gravity-driven sediment deformations controlled by glacial-interglacial and long-term variations in accommodation and sedimentation, have led to the cyclic evolution of the plumbing systems and associated seeping structures since the Plio-Pleistocene.

Source-Parameter Estimation after Attenuation Correction through the Use of Q Tomography

ABSTRACT The measurement of earthquake source parameters is affected by large uncertainties, and different approaches lead to large variability in results. One crucial aspect is the trade-off between attenuation (Q) and corner frequency (fc) in spectral fitting: The source corner frequency, inversely proportional to the fault size, can be severely masked by attenuation and site effects. In this article, we describe a method to solve the trade-off based on the fit of displacement spectra to find the source characteristics (corner frequency, fc, and the signal moment, Ω0) and the single-station attenuation operator (t*), in addition to the site response. We follow a parametric approach based on the use of 3D Q seismic tomography and a bootstrap-based method for selecting the best spectra fit. The correction of attenuation with synthetic values derived by 3D attenuation tomography efficiently deals with the trade-off between source and path terms, leading to small uncertainties in the determination of source unknowns (fc and signal moment, Ω0), thus yielding constrained estimates of source parameters for low- to medium-magnitude earthquakes. We show an application to the Emilia 2012 seismic sequence, for which we computed the source parameters for 1240 aftershocks (from an initial dataset of 1748) with local magnitude ranging from 2.0 to 4.7 using the spectral fit from P and S waves. About 80% of stress-drop estimations are characterized by relatively low uncertainties (within 20% of the estimated values), with maximum values of about 40% for the remaining 20%. The attenuation correction is effective to determine source parameters for small-magnitude earthquakes; hence, we obtain reliable estimates of source parameters for the entire aftershock sequence. This approach gives the opportunity to infer the mechanical state of a complete fault system by taking advantage of the larger number of low-magnitude events (with respect to the largest ones) that always follow a major earthquake.

Silurian-Holocene tectonostratigraphy of Abu Dhabi, United Arab Emirates

The Emirate of Abu Dhabi is located south of the Arabian Gulf and is home to several prolific oil- and gas fields. The stratigraphic column is up to 15 km thick and reflects a complex history of extensional and compressional deformation since the Silurian. Here, we integrate sizable 2D seismic reflection profiles, 3D seismic volumes, and data from over 500 exploration wells to investigate the Silurian-Holocene tectonostratigraphic framework of Abu Dhabi and the adjacent areas. Seven seismic sequences were identified from well-log properties, seismic reflection configurations, reflector geometries, and reflection terminations: Silurian, Devonian-mid-Permian, Upper Permian-Jurassic, Lower Cretaceous-Turonian, Upper Cretaceous, Paleocene-Eocene, and Oligocene-Holocene. The sequences include regional-scale unconformities that can be linked to the tectonic evolution of the Arabian Plate. Four tectonostratigraphic units were defined by distinct seismic sequences, regional stratigraphic gaps, and plate-wide tectonic settings including intracratonic, back-arc to rift margin, passive margin, and compressional phases. In addition to basins with a tectonic origin, four intrashelf basins developed in the area at various times due to a combination of climatic and eustatic variations with differential intraplate subsidence. These basins encompass important hydrocarbon habitats in the basinal and organic-rich deposits of the Upper Jurassic Diyab, Lower Cretaceous Habshan and Shuaiba, and Upper Cretaceous Shilaif formations. An improved understanding of the tectonostratigraphic framework of Abu Dhabi facilitates hydrocarbon exploration endeavors by ascertaining the hydrocarbon prospectivity of deep plays in the region.

Deep well new data in the area of the 2022 Mw 5.5 earthquake, Adriatic Sea, Italy: In situ stress state and P-velocities

Following the Mw 5.5 earthquake of 9 November 2022 that occurred along the Adriatic coast of Italy and the related seismic sequence, we analysed new data from deep wells drilled in recent years by ENI S.p.A. Specifically, we calculated the horizontal stress orientations from breakouts recognized in three wellbores, determined the P-wave velocities for the first 5 km of crust from 12 well sonic records, and then calculated the vertical stress values in the area. Our results, indicating orientations of the present-day stress field of ∼N130 and ∼N040 for minimum and maximum horizontal stress, respectively, are consistent with the seismic sequence focal solutions. Thrust faulting focal solutions occurring at a hypocentral depth between 5 and 10 km, show minimum horizontal stress oriented at ∼N140, which is also in agreement with the compressive tectonics of the area and the known seismogenic sources. The crustal P-wave velocities vary as a function of both the lithology crossed and the geological-structural location of the wells, suggesting several velocity patterns in a confined area. The average vertical stress for the investigated area, inferred from the velocity-derived density, shows values of ∼100 MPa at a depth of 4.5 km, with a gradient varying from 21 to 23 MPa/km. The obtainedin situstress measurements indicate that no stress regime changes or temporal rotations have been observed in the past 50 years throughout the entire area. The new stress indicators fill a geographical gap between the northern and southernmost existing data coherently with a compressive stress regime.

Failures and damages of historical masonry structures induced by 2012 northern and 2016–17 central Italy seismic sequences: Critical issues and new perspectives towards seismic prevention

The work analyses some observed structural criticalities in historical masonry structures emphasized by the seismic events of May 2012, in Emilia, and of August-October 2016, in Central Italy. The causes that brought to different damage scenarios in the historic centres and to the different degree of damage evolution of the buildings, have been investigated, comparing their construction and dynamic features, such as the natural vibration periods of the different construction typologies with the spectral accelerations and the amplification periods distinctive of the seismic events. In particular, starting from the evidence of the damage observed on the built heritage of some of the centres most affected by the two seismic sequences, and from the characteristics of the seismic signals recorded at the considered sites, it is proposed a study which analyses some criticalities, in relation to preventive purposes of the technical standards, specifically with reference to historical constructions. Among the main criticalities highlighted in the present work, it should be noted the remarkable difference between the design spectra and the spectra of the recorded seismic events (real earthquakes), and the circumstance that in reality the constructions generally are interested by more seismic shocks (multi-events) characterized by higher intensities with respect to the ones expected by the building code. Such feature gives rise to a criticality for the operators acting in the emergency phases of rescue and temporary securing, who find themselves operating in high hazardous conditions, due to the high vulnerability of already damaged built heritage induced by the main-shock, and by the high probability (near certainty) that, in the short period, events having intensities and characteristics similar to the main-shock may occur. Moreover, the evolution of the damages, with progressive crumbling associated to global or partial collapses, due to the sequence of shocks (multi-events), which had interested thousands of buildings in the historical centres (and not only) subjected to the seismic sequence, bare witness of criticalities in the preventive approach proposed at national level. The analysis of the criticalities highlighted in the present work allows to envisage an approach which considers for the structures, historical or not, adequate performance requirements with respect to the emergency, temporary and permanent securing phases, accounting for the multi-event seismic context typical of the Italian territory.

Scattering Attenuation Images of the Control of Thrusts and Fluid Overpressure on the 2016–2017 Central Italy Seismic Sequence

AbstractDeep fluid circulation likely triggered the large extensional events of the 2016–2017 Central Italy seismic sequence. Nevertheless, the connection between fault mechanisms, main crustal‐scale thrusts, and the circulation and interaction of fluids with tectonic structures controlling the sequence is still debated. Here, we show that the 3D temporal and spatial mapping of peak delays, proxy of scattering attenuation, detects thrusts and sedimentary structures and their control on fluid overpressure and release. After the mainshocks, scattering attenuation drastically increases across the hanging wall of the Monti Sibillini and Acquasanta thrusts, revealing fracturing and fluid migration. Before the sequence, low‐scattering volumes within Triassic formations highlight regions of fluid overpressure, which enhances rock compaction. Our results highlight the control of thrusts and paleogeography on the sequence and hint at the monitoring potential of the technique for the seismic hazard assessment of the Central Apennines and other tectonic regions.

A Reliable Procedure to Estimate the Rupture Propagation Directions from Source Directivity: The 2016–2018 Central Italy Seismic Sequence

Abstract We present a new approach to estimate the predominant direction of rupture propagation during a seismic sequence. A fast estimation of the rupture propagation direction is essential to know the azimuthal distribution of shaking around the seismic source and the associated risks for the earthquake occurrence. The main advantage of the proposed method is that it is conceptually reliable, simple, and fast (near real time). The approach uses the empirical Green’s function technique and can be applied directly to the waveforms without requiring the deconvolution of the instrumental response and without knowing a priori the attenuation model and the orientation of the activated fault system. We apply the method to the 2016–2017 Amatrice-Visso-Norcia high-energy and long-lasting earthquake series in central Italy, which affected a large area up to 80 km along strike, with more than 130,000 events of small-to-moderate magnitude recorded until the end of August 2022. Most of the selected events analyzed in this study have a magnitude greater than 4.4 and only four seismic events have a magnitude in the range of 3.3–3.7. Our results show that the complex activated normal fault system has a rupture direction mainly controlled by the pre-existing normal faults and by the orientation of the reactivated faults. In addition, the preferred direction of rupture propagation is also controlled by the presence of fluid in the pre-existing structural discontinuities. We discuss the possible role of fluids as a cause of bimaterial interface. Another important finding from our analysis is that the spatial evolution of seismicity is controlled by the directivity.

A Reliable Procedure to Estimate the Rupture Propagation Directions from Source Directivity: The 2016–2018 Central Italy Seismic Sequence

A Reliable Procedure to Estimate the Rupture Propagation Directions from Source Directivity: The 2016–2018 Central Italy Seismic Sequence

Concealed Canyons Delineation and Depositional Elements Analysis Applying a Multidisciplinary Geological and Geophysical Approach: A Case Study of the Pliocene Turbidites, West El-Burullus Area, Offshore Nile Delta, Egypt

A detailed geological-depositional model and a clear understanding of the depositional elements of reservoirs decrease the drilling risks in the development of oil and gas fields. Building an effective geological model is a challenge in West El-Burullus (WEB) area because of its complicated reservoir, which consists of turbidite incised canyons system of Pliocene shales intercalated with sands. These canyons have occurred due to turbidity currents associated with the basin slope forming Kafr El-Sheikh Formation. Deducting the geological-depositional model and basins architecture is carried out utilizing the integration of seismic geomorphology, seismic facies analysis, sequence stratigraphy, and petrophysical analysis. The available datasets comprise a 3D seismic cube and logs of the WEB-1 well. Analyses of these data revealed various lithological facies relations and their paleo-depositional environments. This study demonstrated that Kafr El-Sheik Formation comprises a third-order sequence of turbidite units in-filling a canyon named WEB canyon with five sedimentary facies components: basal lobe through the incised old canyon, amalgamated channels fill, followed by splays, lobes, levees either proximal or distal facies, then the last plugged channels. Time–frequency spectral decomposition of time slices at the base and the top of the WEB canyon assisted the description of the litho-morphological and facies distribution of the concealed Kafr El-Sheikh Formation turbidity incisions and recognized reservoir trending and the canyon fairway. The turbidity facies with high reservoir quality was found in the overbank sedimentary distribution channels such as proximal levees and splays that reflected good to excellent petrophysical reservoir characteristics.

Dual‐sensor wavefield separation in a compressed domain using parabolic dictionary learning

AbstractIn the marine seismic industry, the size of the recorded and processed seismic data is continuously increasing and tends to become very large. Hence, applying compression algorithms specifically designed for seismic data at an early stage of the seismic processing sequence helps to save cost on storage and data transfer. Dictionary learning methods have been shown to provide state‐of‐the‐art results for seismic data compression. These methods capture similar events from the seismic data and store them in a dictionary of atoms that can be used to represent the data in a sparse manner. However, as with conventional compression algorithms, these methods still require the data to be decompressed before a processing or imaging step is carried out. Parabolic dictionary learning is a dictionary learning method where the learned atoms follow a parabolic travel time move out and are characterized by kinematic parameters such as the slope and the curvature. In this paper, we present a novel method where such kinematic parameters are used to allow the dual‐sensor (or two‐components) wavefield separation processing step directly in the dictionary learning compressed domain for 2D seismic data. Based on a synthetic seismic data set, we demonstrate that our method achieves similar results as an industry‐standard FK‐based method for wavefield separation, with the advantage of being robust to spatial aliasing without the need for data preconditioning such as interpolation and reaching a compression rate around 13. Using a field data set of marine seismic acquisition, we observe insignificant differences on a 2D stacked seismic section between the two methods, whereas reaching a compression ratio higher than 15 when our method is used. Such a method could allow full bandwidth data transfer from vessels to onshore processing centres, where the compressed data could be used to reconstruct not only the recorded data sets, but also the up‐ and down‐going parts of the wavefield.

The aftermath of the Emilia 2012 earthquake damages and the safeguard of the rural anthropized landscape

In seismic-prone areas with spontaneous built heritage, extensive measures are needed to protect critical tangible features and intangible values made of vernacular construction techniques. Operating strategies should be continuously improved, starting with the damage survey and moving through the provisional safety measures to the final intervention. An example of the methodology used to preserve the distinctive values of the anthropized countryside landscape in the Emilia lowland is illustrated in this paper by identifying spontaneous evidence derived from the practical knowledge of rural cultures. Through an analysis of historic construction methods, it is possible to comprehend informal architecture by exploring brick masonry constructions, as well as their variances. Although their vulnerability is attributed to the lack of materials and modest construction methods, spontaneous architecture, far from being endangered as the result of seismic sequences, contributes to the distinctive nature of the anthropized rural landscape. In addition, preserving spontaneity increases collective memory and the image of a time.

The 2023 Southeast Türkiye Seismic Sequence: Rupture of a Complex Fault Network

Abstract On 6 February 2023, southeastern Türkiye experienced two Mw 7.7 and 7.6 earthquakes. The earthquake sequence caused widespread damage and tens of thousands of casualties in Türkiye and Syria. We analyze mainshocks and aftershocks, combining complementary source characterization techniques, relying on local, regional, and teleseismic data. Backprojection analysis and finite source inversion for the mainshocks resolve coseismic slip, rupture length, and propagation mode along the main faults, whereas centroid moment tensor inversion for 221 aftershocks resolves details of the fault network. The first mainshock nucleated on a splay fault and activated the neighboring East Anatolian fault zone (EAFZ). It ruptured bilaterally along ∼500 km first toward northeast and later to south-southwest on multiple, previously partly dormant fault segments. The second mainshock ruptured the east–west-oriented Sürgü-Misis fault zone (SMFZ), reaching a slip of 7 m. The analysis of aftershocks with heterogeneous moment tensors retrospectively reconstructs rupture details. Along the main strand of the EAFZ, they map the geometry of different segments in unprecedented detail, whereas along the SMFZ they illuminate the geometry and behavior of large structures for the first time. Our work sheds light on multiple aspects of rupture evolution and provides new insights into the devastating earthquake sequence.

Seismic sequence stratigraphy and depositional modelling of the Lower Fahliyan Formation in the northwestern Persian Gulf, SW Iran

This study presents seismic stratigraphic-based sedimentary conceptual modelling through the integration of well data, seismic data, and literature in order to identify the architecture of the Berriasian-Early Valanginian Lower Fahliyan Formation in the northwestern Persian Gulf. The applied methodology is configured by interpreting seismic sections in terms of sequence stratigraphy and adapting them using well data facies associations and fossil assemblages. The time framework of these sediments is identified by their faunal/flora assemblage and extrapolated by seismic data because of their extensive coverage. Three third-order depositional sequences (DS-I, DS-II, and DS-III) were distinguished based on the facies changes, depositional geometries, and fossil assemblages. Regional and local tectonic activities and global sea-level changes controlled the sedimentation of the depositional sequences. The local tectonic activities were of higher importance as they strongly affected the thickness and facies type of the studied interval. The depositional sequences of the Lower Fahliyan Formation are deposited in the inner to outer settings of a carbonate shelf. It was concluded that DS-I and DS-II over the platform (Surmeh Platform), consisting of progradational parasequences, deposited during the highstand phase in shoal settings that due to primary texture and subsequent diagenetic imprints yield the best reservoir quality. The relationship of the parasequences with good reservoir quality was determined on seismic sections and the prepared regional correlation panel over the study area. Furthermore, the paleoenvironmentof the area was constructed and modelled for major stratal surfaces (Sequence boundaries and maximum flooding surfaces). According to the generated models, the distal setting (Garau Basin) migrated back gradually northward during the Berriasian-Early Valanginian so that it was completely replaced by a homoclinal ramp at the end of the DS-III deposition. Subsequently, the relief of Surmeh Platform was also flattened and it was recognized as the end of the first phase of the basin's activity.

Real time Gutenberg–Richterb-value estimation for an ongoing seismic sequence: an application to the 2022 marche offshore earthquake sequence (ML 5.7 central Italy)

SUMMARYWe estimate the b-value parameter of the Gutenberg–Richter law for earthquake magnitudes in the early stage of the Costa Marchigiana (Italy) seismic sequence, starting on 2022 November 9, with an ML 5.7 event in the Adriatic sea. In particular, we estimate both the completeness magnitude Mc and the b-value within the first 4 and 7 d after the initial strong event in the sequence. Our work represents a practical example of b-value estimation in ‘true’ real time, that is, during the seismic sequence, and its possible interpretation in terms of short-term forecasting. We highlight some critical issues to consider both in estimating/intepreting the b-value, and in evaluating the real time estimation of Mc. These issues are mainly due to the fact that preliminary catalogues available in real time are quite different from the revised ones, which are usually delivered after a few months. The criticalities are linked to the raw data recorded at an early-stage, an unreliable evaluation of the Mc with statistical approaches, the Short Term Aftershock Incompleteness entailed after the initial strong event, and the magnitude binning. Our results show that real time estimation of the b-value can give insights into the evolution of an ongoing seismic sequence, when attention is paid to data quality and quantity.

Bayesian modeling of the temporal evolution of seismicity using the ETAS.inlabru package

The epidemic type aftershock sequence (ETAS) model is widely used to model seismic sequences and underpins operational earthquake forecasting (OEF). However, it remains challenging to assess the reliability of inverted ETAS parameters for numerous reasons. For example, the most common algorithms just return point estimates with little quantification of uncertainty. At the same time, Bayesian Markov chain Monte Carlo implementations remain slow to run and do not scale well, and few have been extended to include spatial structure. This makes it difficult to explore the effects of stochastic uncertainty. Here, we present a new approach to ETAS modeling using an alternative Bayesian method, the integrated nested Laplace approximation (INLA). We have implemented this model in a new R-Package called ETAS.inlabru, which is built on the R packages R-INLA and inlabru. Our study has included extending these packages, which provided tools for modeling log-Gaussian Cox processes, to include the self-exciting Hawkes process that ETAS is a special case of. While we just present the temporal component here, the model scales to a spatio-temporal model and may include a variety of spatial covariates. This is a fast method that returns joint posteriors on the ETAS background and triggering parameters. Using a series of synthetic case studies, we explore the robustness of ETAS inversions using this method of inversion. We also included runnable notebooks to reproduce the figures in this article as part of the package's GitHub repository. We demonstrate that reliable estimates of the model parameters require that the catalog data contain periods of relative quiescence, as well as triggered sequences. We explore the robustness of the method under stochastic uncertainty in the training data and show that the method is robust to a wide range of starting conditions. We show how the inclusion of historic earthquakes prior to the modeled time window affects the quality of the inversion. Finally, we show that rate-dependent incompleteness of earthquake catalogs after large earthquakes have a significant and detrimental effect on the ETAS posteriors. We believe that the speed of the inlabru inversion, which includes a rigorous estimation of uncertainty, will enable a deeper exploration of how to use ETAS robustly for seismicity modeling and operational earthquake forecasting.

Instantaneous limit equilibrium back analyses of major rockslides triggered during the 2016–2017 central Italy seismic sequence

Abstract. Among the almost 1400 landslides triggered by the shocks of the 2016–2017 central Italy seismic sequence, only a limited number, all classifiable as rockslides, involved volumes larger than 1000 m3. Four of these failures, including the three largest among the documented landslides, were described in terms of structural and geomechanical investigations in a previous paper. In this study, the estimated acceleration time histories at the rockslide sites were evaluated through a 2D simplified numerical model accounting for the attenuation phenomena and for the topographic effect of the rock cliffs from which the slide detached. Instantaneous stability analyses were carried out to obtain insights into the variability of the instantaneous margin of safety along the motion, over the entire spectrum of mechanisms that could be activated. Finally, some general suggestions on the pseudo-static verification method for 3D cases are proposed, which represent useful indications to hazard evaluation at local and regional scales.

The temporal evolution of induced seismicity sequences generated by low-pressure, long-term fluid injection

Traffic light schemes (TLSs) are commonly used to mitigate induced seismicity caused by subsurface fluid injection. Verdon and Bommer (J Seismol 25:301–326, 2021a) investigated the validity of the implicit assumptions that underpin the successful functioning of TLSs. In particular, they examined the extent to which magnitude jumps (sudden increases in event magnitudes from any preceding seismicity) and trailing events (continued increases in seismicity levels after the end of injection) took place in hydraulic fracturing (HF) induced seismicity sequences. Other technologies such as carbon capture and storage (CCS), wastewater disposal (WWD) and natural gas storage (NGS) involve the gradual but long-term injection of large fluid volumes at low pressure. Hence, we might expect to see a different spatial and temporal evolution of magnitudes for seismicity induced by low-pressure, long-term (LPLT) injections compared to HF. In this study, we compile cases of LPLT injection-induced seismicity in order to examine their temporal evolution. We examine the occurrence of magnitude jumps, trailing events and onset times for seismicity after the initiation of injection. We find that few LPLT injections have produced trailing events, and that magnitude jumps are typically below 1.5 magnitude units. The timescale of event occurrence (relative to the onset of injection) is highly variable, likely reflecting site-specific conditions. For long-term injection, we observe a trend for the largest events to occur within the earlier part of each sequence, with magnitudes then stabilising, or even reducing, as injection continues. Finally, we evaluate the performance of the next record breaking event (NRBE) model as a method for forecasting induced event magnitudes, finding that this method performs reasonably well in most cases, but that in some cases the largest event significantly exceeds this model.